Downton Abbey will break from tradition when the new series opens later this year.

Not for series four the Labrador's bottom in the opening credits. This time we begin with the midnight flit of one of the main characters. The servants are seen scurrying to and fro, gossiping about the sudden departure as the kitchen buzzes to the sound of a new-fangled electric food mixer: the unstoppable force of progress making itself felt despite the Crawley family's continuing troubles.

It is 1922 and the family are still black-clad and mourning Downton's recently deceased heir, Matthew Crawley. We rejoin them six months after the car crash that killed off the heart-throb husband of Lady Mary (Michelle Dockery) during the show's Christmas day episode, which was watched by over seven million viewers. We know six months have passed because the dowager countess (Maggie Smith) is informed that a grave takes six months to settle before a headstone can be placed over it.

Exposition continues to play a large part in Julian Fellowes's oddly anachronistic dialogue. At several points in the first episode characters explain that they "choose life", must "sort him out" or that they "had a hunch" about something. Nevertheless, the impeccable timing of Smith's verbal smackdowns continues to buoy proceedings with moment after moment of sublime comic satisfaction.

The presence of Smith is even more crucial this series as newly widowed Lady Mary haunts the corridors of Downton like a sulky wraith, dropping lines from her barely open mouth like great leaden plums. Dockery's portrayal of the icy aristocrat finally fits with her stony countenance as she gets to play genuine sorrow at the sudden loss of her husband.

Executive producer Gareth Neame said the show had seen a return to "watercooler, must-see, in-the-moment drama" which lived and died on its narrative surprises and promised more for the new series. If there's one thing Downton producers can do it's keep a secret – with the exits of major characters including Lady Sybil (Jessica Browne Findlay) last series often coming as a complete surprise to its audience.

As some depart, new faces will appear, with guest stars this series including James Fox, Gary Carr as the show's first black character, and Hollywood actor Paul Giamatti, who will play Lady Cora's wayward brother in the Christmas day episode, alongside Shirley MacLaine, who returns as Cora's outspoken mother.

Another guest star, Dame Kiri Te Kanawa, made a surprise appearance at the press launch to sing two pieces that she will perform in episode three, playing opera singer Dame Nellie Melba who visits the family seat for a house party.

She said she had been thrilled to receive the call from the show's producers and admitted, "I couldn't say yes fast enough". She has since named one of her dogs Abbey in deference to her "most favourite programme in the whole world".

With no immediate political upheaval, social change or war on the horizon, the story returns to the domestic as servants and gentry agonise over estate management, staffing quibbles and, at one point, a dropped mixing bowl.

But producers say they were still thinking big in terms of scale for the fourth series. Executive producer Liz Trubridge said: "There's no doubt that we are getting bigger and bigger in terms of our ambition. We've just done something which we never thought we could pull off in the way we have."

She wouldn't be drawn on what it will involve but in past series the show has tackled, off-screen at least, the sinking of the Titanic and the first world war.

Her fellow executive producer Gareth Neame enthused that "the show is in very, very strong health" and said it was as "fresh, vibrant and exciting as the first series". The period drama, which he described as a "phenomenon", has now sold to over 220 territories around the world and is believed to have been seen by approximately 120 million people since its launch in 2010.

Neame added: "It's almost like watching a horror movie that scares you and stimulates your senses, but you come back for more and you enjoy it."

Whether viewers will keep coming back for more after this run is yet to be confirmed with producers yet to confirm a fifth series.